linux-kernel - Re: [PATCH 08/10] drm/i915/reg: replace __is_const_expr() by is_const_true() or is

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Message-ID: <CAMZ6Rq+XhOb+yn5A1dHD=qkbB1_FQXMA7_ydBB4nPTSnys3jkA@mail.gmail.com>
Date: Fri, 6 Dec 2024 00:56:35 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: David Laight <David.Laight@...lab.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Yury Norov <yury.norov@...il.com>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Kees Cook <kees@...nel.org>, 
	"Gustavo A. R. Silva" <gustavoars@...nel.org>, Jani Nikula <jani.nikula@...ux.intel.com>, 
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>, 
	Tvrtko Ursulin <tursulin@...ulin.net>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Mike Leach <mike.leach@...aro.org>, James Clark <james.clark@...aro.org>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Rikard Falkeborn <rikard.falkeborn@...il.com>, 
	Martin Uecker <Martin.Uecker@....uni-goettingen.de>, 
	"linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>, 
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>, 
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"coresight@...ts.linaro.org" <coresight@...ts.linaro.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 08/10] drm/i915/reg: replace __is_const_expr() by
 is_const_true() or is_const()

On Thu. 5 Dec 2024 at 04:00, David Laight <David.Laight@...lab.com> wrote:
> From: Vincent Mailhol
> > Sent: 02 December 2024 17:34
> >
> > Most of the use of __is_const_expr() in i915_reg_defs.h are just to
> > test whether an expression is known to be true. Because those checks
> > are all done in a BUILD_BUG_ON_ZERO(), replace those with
> > is_const_true().
>
> Another place that could use statically_true() and BUILD_BUG_ON_MSG().

Here also, BUILD_BUG_ON_MSG() is not suitable because it does not
return a value.

__BUILD_BUG_ON_ZERO_MSG() could be used; but there is less benefit to
do this at a driver scope. In this i915_reg_defs.h,
BUILD_BUG_ON_ZERO() is used 20 times. Adding an error message each
time will just make things ugly.

If we want more readable error messages here, the solution for me is
just to redefine BUILD_BUG_ON_ZERO() to print a more meaningful error
message by default. But this is not the scope of this series. I sent a
separate patch for this:

  https://lore.kernel.org/all/20241205151316.1480255-2-mailhol.vincent@wanadoo.fr/

Concerning statically_true() instead of is_const_true(), let me test,
and if this works, then I will replace these in v2.


Yours sincerely,
Vincent Mailhol